Helicopters, particularly in deceleration from normal flight, experience high oscillatory forces which generally are attributable to the helicopter blades becoming excited by reaction with blade wake vortices. More particularly, during deceleration, the blades overrun turbulent air. The result is that high vibrations are generated. The principal force generally is found to be at a frequency of N per revolution (N per rev), where N is the number of blades in the rotor. The higher harmonics of N per rev may also be present. However, the principal problem has to do with the N per rev oscillatory force, and the resulting vibrations.
Heretofore, various types of systems have been employed in attempting to minimize the unwanted oscillations involving decreasing the transmissibility from the pylon to the helicopter fuselage even though the oscillatory vertical shear (tension and compression) in the mast increases. Transmissibility may be understood to be the ratio of the forces transmitted to the fuselage, divided by the force at the hub that is generated by the rotor.
Prior art systems have been adversely affected by system damping, spring rate changes, RPM variations and lack of rigidity of the airframe structure in the pylon mounting area.